Phosphate coating of metals



United States Patent C "PHOSPHATE COATING F METALS Montagu Hyams, London, England, asslgnor, by direct and mesne assignments, to Neilson Chemical Company, Detroit, Mieh., a copartnership No Drawing. Application July so, 1956 Serial No. 600,657

Claims priority, application Great Britain August 9, 1955 9 Claims. (Cl. 148--6.16

This invention relates to the phosphate coating of metals.

It is known to produce upon the surface of metals, in

particular ferrous metals, a protective or paint-bonding adherent coating comprising ferrous phosphate. Thus the metal may be coated using an aqueous solution comprising as active coating ingredient a water-soluble orthophosphate of ferrous ironer of an alkali metal, ammonium or an organic base. It is the usual practice, after the formation of such phosphate coatings, to rinsethe coated articles in water and then in dilute chromic acid or aqueous solution containing hexavalent chromium.

I have now found that the ferrous phosphate-com taining coating becomes hydrolysed and oxidised to a considerable extent during the intermediate water rinse, so that during the subsequent chromic acid rinse it cannot react as effectively with the chromic acid to form a chemical compound therewith of suitable corrosioninhibiting power as is the case when the freshly formed phosphate coating is immersed directly in chromic acid. I have found that it is advantageous for the phosphated articles to be immersed directly in the chromic acid rinse after leaving the phosphating bath, or to be immersed in a non-hydrolysing (preferably reducing) intermediaterinse such as a dilute solution of a non-oxidising acid containing a reducing agent, prior to the chromic acid treatment.

I have also found that if the ferrous phosphate containing coatings are immersed, directly after leaving the phosphating bath, in an aqueous solution containing an adequate concentration of metal cation the orthophosphate of which is less soluble than that of ferrous ion, then chemical interchange takes place whereby some of the less soluble phosphate is introduced into the coating. Preferably this less soluble phosphate is more stable than ferrous phosphate. Thus immersion of the coated article in dilute zinc dihydrogen orthophosphate containing hydroxylamine phosphate will convert part at least of the ferrous phosphate or basic ferrous phosphate of the coating into zinc phosphate. The coated article may then be immersed in a conventional chromic acid rinse, whereby to form a more favourable chromate compound than of iron, in the example, zinc chromate. It is not necessary to have separate zinc phosphate and chromate rinses, since a dilute solution of zinc dihydrogen orthophosphate containing chromic acid may be used, the use of a reducing agent being impracticable and unnecessary. Where a single chemical rinse is applied, a subsequent water rinse may be advisable.

The process of the invention comprises treating a ferrous surface successively in the stated order with an acidic aqueous phosphate coating solution whereby to form an adherent coating consisting at least in part of ferrous phosphate, then with at least one aqueous preliminary rinse solution having apH of less than 4.5 and preferably of less than 3.0, and then with a final acidic rinse solution comprising ions of hexavalent chromium.

2,928,762 Patented Mar. 15, 1960 The preliminary rinse solution preferably comprises ions of a non-oxidising acid such as orthophosphoric acid, whereby the stability and activity of the ferrous phosphate in the coating'is preserved. For this'reason also, the preliminary rinse solution preferably also contains a reducing agent such as hydroxylamine phosphate, and is advantageously used hot since the stability of ferrous phosphate in the presence of an aqueous acidic medium is higher at higher temperatures.

According to a particularly advantageous modification of the invention, the preliminary rinse solution also contains an effective concentration of at least one metal cation the insoluble orthophosphate of which is less soluble and more stable than that of ferrous iron, whereby chemical interchange takes place to form a substantial proportion of the less soluble phosphate in the coating. This phosphate is more stable than ferrous phosphate, so that there is less risk of oxidative degradation of the coating by the chromic acid rinse subsequently applied before this has had a chance to effect transformation to insoluble chromate, the chromate preferably being of lower solubility than the phosphate. The preferred ice . cation is zinc, added as zinc primary phosphate.

The preliminary rinse may also contain ions of hexavalent chromium, so that conversion of the coating to chromate may take place at once with no risk of hydrolysis, the final rinse serving to reduce the residual unreacted hexavalent ion concentration, since this should not be excessive otherwise staining and deterioration of subsequently applied paint may result. It is of advantage however to be able to use initially a high chromic acid concentration for a short period to accelerate conversion to the chromate. The final rinse may in fact merely be a water rinse containing chromic acid carried over by the coating, and may also contain a neutralising agent for the chromic acid such as a compound ofzinc. The preliminary rinse may contain as much as 47% hexavalent chromium expressed as CrO Where hexavalent chromium is present in the preliminary rinse, the final rinse containing hexavalent chromium may be omitted, or may be replaced by a rinse not containing hexavalent chromium.

I claim:

1. Process which comprises treating a ferrous surface successively in the stated order with an acidic aqueous phosphate coating solution whereby to form an adherent coating consisting at least in part of ferrous phosphate, at least one aqueous preliminary non-oxidizing rinse of a non-oxidizing acid solution having a pH of less than 4.5 and a final acidic aqueous rinse solution comprising ions of hexavalent chromium.

2. Process as claimed in claim 1, wherein the preliminary rinse solution comprises ions of the non-oxidising acid, orthophosphate acid.

3. Process as claimed in claim 1, wherein the preliminary rinse solution contains a reducing agent comprising a salt of hydroxylamine.

4. Process asclaimed in claim 1 wherein the preliminary rinse solution contains an effective concentration of at least one metal cation the insoluble orthophosphate of which is less soluble and more stable than that of ferrous ion, whereby chemical intercharge takes place to form a substantial proportion of the less soluble phosphate in the coating.

5. Process as claimed in claim 1, wherein the preliminary rinse solution contains an effective concentration of zinc primary phosphate.

6. Process as claimed in claim 1, wherein the preliminary rinse solution comprises ions of orthophosphoric acid and also contains hydroxylamine.

7. Process which comprises the successive steps of a dilute aqueou's ;phosphoric acidtsolution-containing ions of 'zinc in solution-arid'having a pH of le'ss than 3.0, andthen rinsingt he coatinginan acidic aqueous solut-ion comprising ions ornament chromium.

8. P'recess -'wh'ich comprises treating a metal surface with an acidic aqueous.15hosphate coating solution to form an adherent coating consisting at least in part of ferrous-phosphate, applying to said coated surface at least one aqueous. preliminary non-oxidizing rinse and subsequently applying a acidic aqueous rinse solution comprisingions of hexavalentchromium, said preliminary non-oxidizing "rinse; consi sting -of a non-oxidizing acid solution having a.pI-I of-less than 4.5 containing anions of orthophosphoric' acid and cations of zinc the insoluble -or thophosphate -of which is less soluble and more stable than that of ferrous iron.

9. Process which comprises treating a metal surface with --an acidic -aqueous-.-phosphate coating solution --to form anadherent coating consisting at least in part of "fr'fous"pliospliate, "anfil'yin'g to said coated surface at nrrences enea 'in the fiiem this-patent UNITED STKTESWKTENTS 2,293,779 Tanner c .l- Aug. 25, 1942 2,298,280 Clifford can. Oct. 13, 1942 S--LJJ-x-L-a-aa ii- -Spt= 7-, 2,403,426 Doutyetal. July 2, 1946 2,540,314 Amundsen Feb. 6, 1951 2,813,815 Miles Nov. 19, 1957 "FORE'I'GNPATENTS Great .t. Jan. 876,939 Germany May 18, 1953 1',073,146 May 17,

-'-France -2 1954 

1. PROCESS WHICH COMPRISES TREATING A FERROUS SURFACE SUCCESSIVELY IN THE STATED ORDER WITH AN ACIDIC AQUEOUS PHOSPHATE COATING SOLUTION WHEREBY TO FORM AN ADHERENT COATING CONSISTING AT LEAST IN PART OF FERROUS PHOSPHATE, AT LEAST ONE AQUEOUS PRELIMINARY NON-OXIDIZING RINSE OF A NON-OXIDIZING ACID SOLUTION HAVING A PH OF LESS THAN 4.5 AND A FINAL ACIDIC AQUEOUS RINSE SOLUTION COMPRISING IONS OF HEXAVALENT CHROMIUM. 